I have updated and tested, there is two things in the administration which seams odd:

variable visibility in administration

I have a first vhost using uid as namespace and a second vhost using a custom string
The first vhost is running the Administration, he can see his own variable (without namespace) but he can also see variables of the second vhost, with namespace.
This lead to privacy loss. Each administration must be constrains to their namespace, IMHO.

there is a "\0" between the namespace and the variable name.

See attached screenshot.

Regarding the xcache_set_namespace API, how does it work ?
I must be sure the user can't alter his namespace.

about \0: the \0 is intended to solve some stupid domain name introduce by some stupid country. e.g.: example.com vs example.com.cn (while it should be example.co.cn), without \0 separator, xcache_set(".cn", 1) under "example.com" is identical to xcache_set("", 1) under "example.com.cn". domain name is not the only problem. same applies if you have uid 1024 and 10241

about xcache_set_namespace: when user set namespace, it's a inner namespace under the current outer namespace. it's similar to hard ulimit vs soft ulimit. user can never break out of the outer namespace

about admin variable list: the current XCache admin page is designed for server administrator, not vhost owner

i haven't get enough time to test all use cases in namespace. any phpt is welcome